This invention relates to control systems, and more specifically, to an electronic system in the form of a controller suitable for embodiment in a pulverizing mill for purposes of controlling the journal loading on the grinding, i.e., pulverizing, rolls of the mill in accordance with the rate of feed to the mill of the material that is to be pulverized therewithin.
An essential component of any steam generation system of the type, which utilizes pulverized coal as a fuel, is the apparatus in which the coal is pulverized so as to render it suitable for such usage. Although the prior art is known to have employed various types of apparatus for purposes of accomplishing coal pulverization, one form of apparatus in particular, which has frequently been used for this purpose, is that commonly referred to as a bowl mill by those in the industry. The bowl mill obtains its name principally from the fact that the pulverization, i.e., grinding, of the coal that takes place therewithin occurs on a grinding surface which in configuration somewhat resembles a bowl.
By way of illustration, reference may be had to U.S. Pat. No. 3,465,971, which is assigned to the same assignee as the present invention, for a showing of a prior art form of bowl mill. This patent contains a teaching of both the nature of the construction and the mode of operation of a bowl mill that is suitable for use for purposes of effecting the pulverization of the coal that is used to fuel a coal-fired steam generator. As taught by this patent, the essential components of such a bowl mill are a body portion, i.e., housing, within which a grinding table is mounted for rotation, a plurality of grinding rolls that are supported in equally spaced relation one to another in a manner so as to coact with the grinding table such that the coal disposed on the surface of the grinding table is capable of being ground, i.e., pulverized, by the rolls, coal supply means for feeding to the surface of the grinding table the coal that is to be pulverized in the bowl mill, and air supply means for providing to the interior of the body portion the air that is required for the operation of the bowl mill.
In order to satisfy the demands of a coal-fired steam generation system of conventional construction for pulverized coal a multiplicity of bowl mills of the type shown in the aforereferenced patent are commonly required to be employed. Further in this regard it is noted that the individual capacity of each of these bowl mills may range up to a capacity of one hundred tons of pulverized coal per hour. In addition to possessing a capability of operating at their maximum capacity, these bowl mills must also have the ability to operate at less than full capacity, i.e., at some percentage thereof, e.g., 25%, 50%, 75%, etc. Accordingly, this fosters a further requirement that the bowl mill be capable of exerting the requisite degree of grinding force regardless of the rate of output at which the bowl mill is operating. Here note is taken of the fact that variations in the output provided from the bowl mill are normally accomplished by varying the amount of coal that is fed to the grinding table, while the speed of rotation of the grinding table is made to remain substantially constant.
The depth of coal that is disposed on the grinding table is a function of the output rate at which the bowl mill is performing. In addition, the depth of coal that is present on the grinding table has an effect on the amount of grinding force being exerted on the coal by the grinding rolls. Obviously, therefore, it is important that if the grinding rolls are to apply the requisite degree of force needed to effect the pulverization of the coal, consideration must be given to the existence of this relationship between the grinding force exerted by the grinding rolls and the depth of the coal on the grinding table.
Originally, the journal loading, which dictates the amount of grinding force that the grinding rolls exert on the coal, was provided through the use of mechanical springs. One arrangement of this type can be found depicted, for example, in the patent which was referred to above previously. In accord with the showing contained in this U.S. patent, each grinding roll is urged towards the surface of the grinding table by means of an adjustable spring. To this end, there is selected for use for this purpose, a mechanical coil spring that possesses the design characteristics desired; namely, a spring that is capable of urging the grinding roll toward the grinding table surface in such a manner that the grinding roll exerts a predetermined grinding force on the coal disposed on the table, when the coal is of a preselected depth on the table.
It was found, however, that there were at least two significant disadvantages associated with the use of mechanical coil springs for purposes of providing the journal loading on the grinding rolls of a bowl mill. The first of these is of relatively recent vintage and has its origin in the fact that the size of the lastest bowl mills is such as to require coil springs that must be capable of exerting tremendous forces. The problem that has surfaced in this regard is not one of design, but rather has to do with the manufacturing of the coil springs. Namely, difficulties have been encountered in regard to satisfying the existing quality assurance standards for such mechanical coil springs. The result has been that these coil springs, as manufactured, do not always embody the specifications that have been set therefor. Accordingly, variations are found to occur as between the grinding forces exerted by each of the grinding rolls in a given bowl mill. In addition, the designed value of the grinding force that has been established for a particular grinding roll is often not attached. Such variations in the amount of grinding force being exerted by the grinding rolls has led to instances wherein the level of performance of the bowl mill is deemed to be unsatisfactory. That is, the coal has not been pulverized properly because the grinding rolls have not exerted the requisite grinding force. This in turn can have an adverse effect on the operation of the entire coal-fired steam generation system.
The other disadvantage from which bowl mills equipped with mechanical coil springs have been known to suffer is the fact that it is very difficult, if not impossible, therewith to adjust the amount of grinding force that the grinding rolls exert on the coal that is being pulverized. The reason for this lies principally in the fact that each coil spring can only be made to have one spring constant. Moreover, once the coil spring has been made to have a certain spring constant, the latter remains essentially fixed from then on. Basically, a two-step procedure is followed in establishing what the spring constant should be for a particular coil spring. Namely, the amount of grinding force that the grinding roll needs to apply to the coal in order to effect the desired degree of pulverization of the latter under a specified set of operating parameters is determined. From this, it is possible to determine what the journal loading should be on the grinding roll in order to have the latter provide such a grinding force. The proper spring constant is then selected, which will enable the establishment by the coil spring of such a journal loading on the grinding roll.
Unfortunately, as alluded to above, the amount of grinding force that a particular grinding roll should exert on the coal is a function of a number of variables, e.g., the output rate at which the bowl mill is operating and concomitantly therewith the depth of coal that is disposed on the grinding table surface, the nature of the coal that is being pulverized, etc. Any change in any of these variables can necessitate an adjustment in the amount of grinding force being applied to the coal by the grinding roll. Thus, for purposes of making the original determination of the spring constant for the coil spring, a particular set of operating parameters are assumed. This assumed set of parameters are designed to most nearly represent those that most frequently will prevail when the bowl mill is operating. In summary, it is possible when utilizing mechanical coil springs to select a spring constant that will enable the grinding rolls to exert an optimum amount of grinding force under a given set of operating parameters. However, any change in these parameters occasioned by the operating requirements of the steam generation system of which the bowl mill forms a part that leads to a need to adjust the grinding force being exerted by the grinding rolls will mean that the bowl mill will be forced to operate in a condition wherein either less or more than the optimum amount of grinding force is being applied by the grinding rolls. This results from the inability to change the spring constant of the coil springs.
In an effort to obviate the disadvantages associated with the use in bowl mills of mechanical coil springs for purposes of establishing the journal loading on the grinding rolls thereof, the prior art has turned as a possible replacement to the employment of hydraulic systems. U.S. Pat. No. 4,002,299 is directed to one arrangement of such a hydraulic system. In accord with the teachings of this patent, a system is provided wherein the grinding rolls have a hydraulic loading applied thereto. More specifically, the hydraulic loading on the grinding rolls is established by means of hydraulic fluid that is fed under pressure to the grinding rolls. Moreover, through the use of a servo system, changes in the hydraulic pressure are automatically effected as the mill output increases or decreases.
Although hydraulically loading the grinding rolls of the bowl mill has obviated the problem discussed above relating to the matter of meeting quality assurance standards in the manufacture of mechanical coil springs, it has introduced a new and different problem. Namely, a characteristic of the servo systems employed in connection with the establishment of the hydraulic loading on the grinding rolls in bowl mills is the susceptibility of such servo systems to the phenomenon known as hunting. Inasmuch as those skilled in the art of servo systems are well acquainted with this phenomenon, it is not deemed necessary to discuss it at length herein. Rather, it is deemed sufficient to merely note that hunting is that phenomenon wherein the servo system in its attempt to effect the establishment of the proper hydraulic pressure continually signals the need for minor corrective adjustments to be made in the hydraulic pressure. To this end, each time the servo system senses a deviation from the desired pressure level, it signals the need for corrective action to be taken. This corrective action instituted by the servo system in turn elicits from the latter the need for a further change. This process, which may go on ad infinitum, is what is referred to herein as the phenomenon of hunting.
In addition to being disadvantageously characterized by virtue of their susceptibility to hunting, as discussed above, the systems that have been suggested to date by the prior art for purposes of hydraulically loading the grinding rolls in a bowl mill have in general also suffered from a further disadvantage. Reference is had here to the fact that as in the case of mechanical coil springs, most, if not all, of the hydraulic systems that have been suggested for use by the prior art in connection with establishing the loading on the grinding rolls in a bowl mill are unsuitable for use for purposes of effecting changes in the amount of grinding force that the rolls exert. That is, the mode of operation of these prior art forms of hydraulic systems is such that they are intended to ensure that a fixed value of hydraulic pressure is continually applied to the grinding rolls in the form of the journal loading thereon.
The difficulty arises here from the fact that as in the case of the spring constant of coil springs, although a particular value of hydraulic pressure may be selected so as to cause the grinding rolls to exert the optimum amount of grinding force for a particular set of operating parameters, as the latter parameters vary in the course of the operation of the bowl mill, the value of the pressure of the hydraulic fluid being fed to the grinding rolls may not necessarily be the same as that which should be present to ensure that the grinding rolls are still exerting the optimum amount of grinding force under this changed set of operating parameters. Moreover, like the spring constant of the coil spring, once the value of the pressure of the hydraulic fluid that is to be supplied to the grinding rolls is established, in accord with the mode of operation of most, if not all, of these prior art forms of hydraulic systems, this value for the hydraulic pressure cannot be changed. That its, changes cannot be effected in the established value for the pressure of the hydraulic fluid in accordance with the need to vary the amount of grinding force that the grinding rolls are required to exert in order to pulverize to the desired extent the coal that is disposed on the grinding table surface.
A need has thus existed in the prior art for a new and improved means for providing the journal loading on the grinding rolls of a bowl mill. Moreover, a need has been demonstrated for a means that would enable the amount of grinding force being exerted by the grinding rolls to be varied as the need therefor may be occasioned by changes in the operating parameters of the bowl mill. Finally, a need has been shown for such a means which would not suffer from the same difficulties that have served to disadvantageously characterize the operation of bowl mills equipped either with mechanical coil springs or a hydraulic system of the prior art type.
It is, therefore, an object of the present invention to provide a new and improved means operable for establishing the journal loading on the grinding rolls of a bowl mill that is suitable for use to pulverize coal.
It is another object of the present invention to provide such a means that is operative to establish a hydraulic loading on the grinding rolls of a bowl mill suitable for use to pulverize coal.
It is still another object of the present invention to provide such a means in the form of an electronic controller that is operative for purposes of effecting control over the hydraulic loading that is applied to the grinding rolls of the bowl mill.
A further object of the present invention is to provide such an electronic controller that is capable of obviating the problem involving the failure to meet quality assurance standards that has served to disadvantageously characterize the mechanical coil springs that have been employed heretofore for purposes of establishing the journal loading on the grinding rolls in a bowl mill.
A still further object of the present invention is to provide such an electronic controller that is capable of obviating the problem involving susceptibility to hunting that has served to disadvantageously characterize the hydraulic systems that have been employed heretofore for purposes of establishing the journal loading on the grinding rolls in a bowl mill.
Yet another object of the present invention is to provide such an electronic controller that enables adjustments to be made in the amount of grinding force being exerted by the grinding rolls in order to compensate for the occurrence of changes in the operating parameters of the bowl mill.
Yet still another object of the present invention is to provide such an electronic controller that is relatively simple to construct and employ, as well as being relatively inexpensive to provide.